This invention relates to purification of tetrachlorosilane used for the manufacture of electronic grade silicon and, more particularly, to a method for removing trace impurities of phosphorus. The method involves contacting liquid tetrachlorosilane with activated charcoal. The process is effective in reducing the phosphorus levels in the tetrachlorosilane to the parts per trillion range. The process can be run as a continuous or batch process, with easy separation of the activated charcoal containing the phosphorus contaminate from the tetrachlorosilane.
Monocrystalline silicon wafers of extremely high purity are required for the manufacture of integrated circuits. The purity of the monocrystalline silicon wafers is one factor that limits the density of the circuits that can be formed on the silicon wafers. Therefore, as attempts are made to increase the density of the circuits on monocrystalline silicon wafers there is a continuing need to reduce the level of impurities in the silicon wafers.
A standard process for producing monocrystalline silicon of high purity involves the chemical vapor deposition of hyperpure silane gas, for example tetrachlorosilane, onto a heated silicon element. The formed silicon ingot is then float zone processed into a monocrystalline rod which can be sliced into monocrystalline silicon wafers appropriate for forming integrated circuits.
Trace contamination in the silane deposition gas is a source of contamination in monocrystalline silicon wafers. Therefore, it is desirable to reduce the trace contaminate levels in the silane deposition gas as low as possible. Many impurities such as iron, copper, and manganese can be removed from the silanes for example, by distillation. However, phosphorus content cannot be reduced to desired levels by simple distillation because phosphorus tends to form compounds with properties similar to the silanes.
Previously proposed methods for the removal of phosphorus from silanes include for example, complexing of the phosphorus compounds with selected transitional metal compounds (Kray. U.S. Pat. No. 4.481.178. issued Nov. 6, 1984): reacting phosphorus impurities with iodine, bromine or chlorine to form high-boiling compounds which can be separated from the silanes (Bradley. U.S. Pat. No. 3,188,168, issued Jun. 8, 1965); the use of zeolite molecular sieves to separate the phosphorus compounds from the silanes (Caswell, U.S. Pat. No. 2,971,607, issued Feb. 14. 1961): and reacting chlorosilanes in the presence of oxygen at a temperature of about 60.degree. C. to 300.degree. C. to form Si--OH species which complex with impurities such as PCl.sub.3 (Darnell et al., U.S. Pat. No. 4.409.195. issued Oct 11. 1983).
The present activated charcoal method offers advantages over these previously described processes in that the present method can easily remove phosphorus from tetrachlorosilane in the parts per trillion range. In addition, the present process can be run at near room temperature and separation of the purified tetrachlorosilane from the activated charcoal can be accomplished simply.
Ogi et al., JP Kokai Pat. No. Hei 2(1990)-153815. describes a method for purification of chloropolysilanes by contacting with activated carbon. Tarancon, U.S. Pat. No. 4,099.936. issued Jul. 11, 1978. describes a multi-stage process where at least one stage can comprise contacting a silane in the gas phase with activated charcoal at a temperature of minus 10.degree. C. to 50.degree. C. to remove impurities.